Could you please do a small CSS change on your websites.
Please mark the SOLVED button as SOLVED in a GREEN Background Color
and ad a second popup box when a Thread should get reopened again.
Right now it is very unclear if a Thread has been resolved or not. There must be a way to see SOLVED all the time even you are scrolling but at least when you see the button still in grey - it invites to click it again to resolve a problem even the text has changed - which gets easily overlooked as it looks the same as the unresolved button.
Marking Resolved in GREEN would make it clear that it has already been solved. Moving the solution to the top would make it even more clearer.
Leaving both in grey will end up in many wrong clicks!